How important are thumbnails?

Do thumbnails on articles play a big role in convincing people to click?

On Youtube they're fairly important, as they give a lot of info on what the video is about. Can the same be said for Substack?

Currently I just use free stock photos that are somewhat related to the topic of my articles. It's quick and easy but I feel it makes my publication look a bit generic...

I'd be interested on hearing your thoughts on the topic:

Question for writers: What type of thumbnails do you use? How much effort do you put into them?

Question for readers: Do you care what the thumbnails look like? Are there instances where you clicked/did not click on an article due to the thumbnail?

Thanks for the input :)